What is a real RNG

[Quanm qiu]

Hello.I am using Cryptopp5.1 on MS Windows 2003.The compiler is MS VC.net 
2003. I want to write a DSASignFile function just like RSASignFile.So I 
write this:
void DSASignFile(const char *privFilename, const char *messageFilename, 
const char *signatureFilename)
{
          AutoSeededRandomPool rng;
          FileSource privFile(privFilename, true, new HexDecoder);
	DSA::Signer priv(privFile);
	
	FileSource f(messageFilename, true, new SignerFilter(rng, priv, new 
HexEncoder(new FileSink(signatureFilename))));
}

But I always get a exception like "NullRNG: NullRNG should only be passed 
to functions that don't need to generate random bytes" when I use this 
function to sign a file.What is wrong?
Thank you very much!